It works just fine for the Medic in SC:BW. As you moved forward, the medic would march to her death. If you "attack moved" with her, she used her autocast ability when able to. It's important to note that both the medic and changling do not have an attack. Maybe the better term is "action move" but it's not as catchy
My opinion seems to be different than yours on this one. I believe it's not broken because it's the same scenario with every unit in the game. To ensure UI familiarity/consistency we should avoid making the changeling an exception. If a unit is issued a "move" command, it should do no other options, including autocasts. If it's issued "Attack move" it should do actions on the way. That's my two cents on the matter at least. ^_^
(And before I get people discussing how the old medic ability wasn't autocast, the same mentality works in WC3 constantly with any autocast unit, including priests.. Attack move = Autocasts work. Manual move = all other actions off.)